Porous media gas burner

ABSTRACT

A porous media gas burner is adapted to operate in a pressurized environment. The burner may have three zones: a mixing zone, an ignition zone and a reaction zone. The burner may be used as a downhole burner in a formation heat treatment method for oil and gas wells.

BACKGROUND OF INVENTION

The present invention relates to a porous media gas burner and methodsfor its use. In particular, the invention relates to a downhole gasburner used in formation heat treatment methods.

Combustion of gases in porous media is a process where a combustiblegaseous mixture is injected into a porous matrix and is combusted withinthe porous matrix. Flames in porous media have higher burning velocitiesand leaner flammability limits than open flames. These effects are wellknown and are the consequence of excess enthalpy combustion.Essentially, heat that is generated in the combustion zone istransferred by radiation and conduction through the solid phase of theporous media to unburned gases. As a result, it is possible to achievetemperatures higher than the adiabatic flame temperature, and theincrease in burning velocities can be significantly higher than the openflame laminar burning velocity for the same mixture in an open space.

Formation heat treatment is a process that is intended to improvehydrodynamic conditions around the wellbore. If formation temperaturesreach an adequate level, blocking water may be vapourized, claystructure may be dehydrated, clay minerals may be partially destroyed,and microfractures may be induced in the formation near the wellbore. Asa result, permeability around the wellbore may be significantlyimproved.

It is known to use a downhole electrical heater in a formation heatingmethod. The electrical heater is placed as close as possible to thetarget zone, and an inert gas such as nitrogen is co-injected throughthe annulus. The temperature of the injected gas may rise to as high as800° C. before entering the formation. However, this method involveslarge energy requirements which makes it cost-prohibitive, particularlywith rising electrical energy costs.

Combustion stimulation is a known technique to promote fluid productionin a formation. A combustion front is initiated in a wellbore by meansof a surface heater or burner and the front is propagated into theformation to a distance of up to about 6 meters. The formation in thiszone is reduced to clean burnt sand, which is very fluid permeable.However, the well casing is subjected to high temperatures, which isundesirable, and there is an elevated risk of explosions or wellburnouts using this technique. It is necessary to maintain wellboretemperatures below 600° C. in order to prevent damage to the liner,which limits the temperature which may be reached in the formation.

Therefore, there is a need in the art for a gas burner which may be useddownhole in a formation heating process.

SUMMARY OF INVENTION

In one aspect, the invention may comprise a gas burner comprising atubular housing adapted to operate in a pressurized environment, thehousing defining an intake opening and a flue opening and comprisingmeans for receiving a supply of fuel and air; a mixing zone where thefuel and air are mixed comprising a packed bed of porous media; anignition zone comprising a packed bed of porous media, and a reactionzone comprising a packed bed of porous media; wherein the pore size ofthe mixing zone and the reaction zone is smaller than a minimumquenching distance of a fuel gas under standard conditions while thepore size of the ignition zone is larger than the minimum quenchingdistance.

In another aspect, the invention may comprise a downhole formationheating system for a wellbore including a well casing, the systemcomprising:

-   -   (a) a gas burner comprising a cylindrical housing defining an        intake opening and a flue opening, the housing comprising means        for receiving a supply of fuel and air; a mixing zone where the        fuel and air are mixed; an ignition zone comprising an igniter        and a reaction zone, each zone comprising a packed bed of porous        media;    -   (b) an igniter for igniting the fuel and air within the gas        burner;    -   (c) fuel and air supply tubing for delivering fuel and air to        the burner; and    -   (d) means for delivering pressurized air or an inert gas in an        annular space between the well casing and the fuel and air        supply tubing.

In another aspect, the invention may comprise a method of heat treatinga formation comprising the steps of:

-   -   (a) inserting a gas burner comprising a cylindrical housing        defining an intake opening and a flue opening, the housing        comprising means for receiving a supply of fuel and air; a        mixing zone where the fuel and air are mixed; an ignition zone        comprising an igniter and a reaction zone, each zone comprising        a packed bed of porous media, into a wellbore;    -   (b) injecting a fuel gas and air into the gas burner to create a        combustible mixture and igniting the mixture to create a        combustion front; and    -   (c) causing the combustion front to travel out the gas burner        and into the formation.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying simplified, diagrammatic,not-to-scale drawings.

FIG. 1 is a schematic representation of one embodiment of the presentinvention.

FIG. 2 is a schematic representation of one embodiment of a formationheat treatment system.

DETAILED DESCRIPTION

The present invention provides for a gas burner and methods of using agas burner. When describing the present invention, all terms not definedherein have their common art-recognized meanings.

In one embodiment, the invention comprises a gas burner as shownschematically in FIG. 1. The burner generally includes a body (10) whichcomprises a mixing zone (12), an ignition zone (14) and a reaction zone(16) where combustion will take place. The body may have any shape. Inone embodiment, the body may be cylindrical or conical or have bothcylindrical and conical sections. The body is preferably lined with aheat-refractory material such as a ceramic liner.

As shown in FIG. 1, the mixing zone (12) and ignition zone (14) comprisecylindrical portions of the tubular body (10) while the reaction zone(16) comprises a truncated conical portion, with an expanding diameteras the reactants flow away from the mixing zone of the inlet end. In oneembodiment, the outlet end diameter of the reaction zone may be abouttwice that of the inlet end. The reaction zone may have a length about 4to 5 times the diameter of the inlet end. The mixing and ignition zonesmay have a length approximately equal to their diameter.

All three zones (12, 14, 16) may be packed with a porous media bedwithin the burner. The packed bed may comprise heat resistant ceramicspheres such as alumina beads or any other suitable particulate materialto create a porous media bed for example, but not limited to,zirconia-alumina composites, silicon carbide, or mullite(alumina-silicon dioxide). As shown in FIG. 1, air or oxygen is providedto the burner in the mixing zone (12), along with the combustible gas.The gases mix in the mixing zone (12) and are ignited in the ignitionzone (14) by means of an igniter (not shown), which may be a small openflame burner or a spark device. Once ignited, the flame front will beallowed to advance into the reaction zone (16).

In one embodiment, the mixing zone (12) may be packed with small sizeparticles so that the pore size in the mixing zone is smaller than theminimum quenching distance (MQD). The second section would the ignitionzone (14) and may be packed with larger size particles so that the poresize is larger than the minimum quenching distance. The size and natureof the reaction zone (16) particles (pore size) would depend on energyand operational requirements, type of fuel gas and operating conditionsand could be of either uniform size or a combination of sizes.

As used herein, the phrase “minimum quenching distance” or “MQD” shallmean the minimum diameter or opening dimension through which a flame maytravel under standard conditions. It may be observed that a flame in amixture within a flammable range will be extinguished if forced topropagate through a constriction. The walls of the constriction exert arepressive influence on the flame. A flame is quenched in a constrictionbecause of two mechanisms which otherwise permit flame propagation: thediffusion of species, and the diffusion of heat. The walls of theconstriction may extract heat and the smaller the restriction, thegreater the surface to volume ratio will be. Similarly, the smaller theconstriction, the greater the number of collisions of the active radicalspecies with the wall, and the greater the number of these species whichare destroyed. Accordingly, one skilled in the art will understand thatincreased temperature decreases the quenching distance and thatquenching distance decreases as pressure increases.

MQD is a physical property of each fuel and may be determined in thelaboratory or by using the criteria of Peclet number equal to 65. ThePeclet number is a dimensionless parameter that is based on the specificheat, laminar burning velocity, density, thermal conductivity andthermal diffusivity of the gas mixture, however the heating of theporous media bed will affect the minimum quenching distance.

It is generally accepted that main driving factor in the combustion ofgases in a porous media is heat recirculation through the porous mediato preheat unburned reactants. This preheating of the reactants maypermit combustion even if the pore size is smaller than the MQD of theporous media under standard conditions.

In one embodiment, the burner (10) is adapted to operate in apressurized environment. The body of the burner (10) is designed towithstand the desired pressure. This provides the opportunity tointegrate the burner in the middle of a process stream, so that exhaustgases may be recovered at pressure for further treatment, separation orother downstream processes. It may allow use in subterranean hydrocarbonformations as will be subsequently described. The porosity of the packedbed may be controlled for specific applications. In one embodiment, thepacked bed in the mixing zone and the reaction zone has a pore sizesmaller than the minimum quenching distance, while the ignition zonepore size may be larger than the minimum quenching distance. The poresize, combustible mixture flux, concentration of fuel gas in oxidant(air), type of fuel and shape of the burner may be varied to permit andoptimize the process in a pressurized environment. A person skilled inthe art may determine these matters with minimal and routineexperimentation.The main effect of operating pressure in the gas burneris a reduction in the combustion front velocity. Maximum temperaturesattainable when operating at elevated pressures are generally lower thanthose temperatures observed for the same gas mixtures and fluxes atatmospheric pressure. While the effect of pore size is almost negligibleat atmospheric operating pressure, as the operating pressure isincreased, the velocity of the combustion front increases as the poresize decreases. At elevated operating pressures, burning velocitiesappear to increase as the inlet gas velocity increases. Additionally,burning velocities appear to increase as the fuel gas concentration isdecreased.

The relatively smaller pore size of the mixing and the reaction zonepromotes mixing of the reactants and pre-heating of the reactants due toheat transfer from the solid phase to the gas phase. The relativelylarger pore size of the ignition zone allows easier ignition of thereactants and propagation of the flame into the reaction zone.

The gas burner (10) may be used as a downhole gas burner to be used in aformation heating method. Generally, the formation heating method maycomprise two stages. In a first stage, the burner is placed in thewellbore at the level of the formation by means of coiled tubing or thelike. The tubing also provides the means by which a combustible mixtureis provided to the burner. The combustible mixture may be a lean mixtureof natural gas and air which is below the flammability limits of naturalgas at atmospheric pressure. The mixture may sustain a combustion frontwithin the burner which expels hot flue gases into the formation. Ifdesired, the combustion front may be controlled to travel outward fromthe burner into the formation. The combustion front is controlled byincreasing or decreasing the fuel gas flux, changing the concentrationof fuel gas in the mixture fuel-air (oxidant) and by using differentparticle size or a combination of particle sizes in the reaction zone.These variable elements may be varied either singly or in combination.

Depending on the composition of the formation hydrocarbons, the amountof oxygen in the flue gas and the flue gas temperature, someoxidation/combustion reactions may start to take place in the formationat the same time as gas combustion is occurring in the burner. Once thecombustion front leaves the burner, it is preferred to increase theconcentration of the fuel, so that the temperature of the reaction frontincreases. This may be done safely because the temperature in the burnerand the wellbore will not be high enough to facilitate or sustaincombustion. In other words, the burner becomes a flame arrester once thecombustion front travels outward into the formation.

The burner described herein may be used as a downhole burner inalternative methods. As described above, the burner may be used in wellstimulation method where blocking water is vapourized, clays may bepartially destroyed, asphaltenic deposits may be burned andmicrofractures in the formation may be propogated. In another example,the downhole gas burner may be used in a downhole steam generationmethod for cyclic or continuous steam injection in deep heavy oil wells.In another example, the downhole gas burner may be used in a highpressure air injection technique, where combustion is initiated in theformation and air is then continuously injected into the producinglayer. The combustion of inplace oil may provide thermal and gas driveto the oil reservoir.

In one embodiment, with reference to FIG. 2, a downhole burner (10) ispositioned near the producing formation (20) by means of continuous orcoiled tubing (22) and anchored to the well casing by means of an anchor(24). Air is injected through the tubing (22) and a combustible gas suchas natural gas is injected through a separate tubing (26) to the burner.Air or an inert gas such a nitrogen may be injected in the annulusbetween the well casing and the tubing (22). The burner has a mixingzone (120), an ignition zone (122) and a reaction zone (124), eachpacked with a suitable porous media. As described above, in a preferredembodiment, the particle size or minimum quenching distances in eachzone may be varied. The combustion front (126) will be established inthe reaction zone (124) and move outward into the formation. At the sametime, hot flue gases (128) are pushed outward into the formation.

In each case, the ignition and combustion downhole and/or in theformation may be initiated using a lean combustible mixture, which mayinclude waste gases. The lean mixture reduces the risk of explosivemixtures accumulating in the system.

As will be apparent to those skilled in the art, various modifications,adaptations and variations of the foregoing specific disclosure can bemade without departing from the scope of the invention claimed herein.The various features and elements of the described invention may becombined in a manner different from the combinations described orclaimed herein, without departing from the scope of the invention.

EXAMPLES

The following examples describe specific embodiments which are exemplaryof the present invention. They are not intended to limit the claimedinvention.

Example 1

Porous Media

In one embodiment, the porous media comprises relatively uniform aluminaspheres (90% alumina and 10% silica) having the following physical andhydrodynamic properties:

-   -   (a) specific heat capacity at 20° C., J/kgK 920    -   (b) thermal conductivity at 20° C., W/mK 16.7    -   (c) Density, kg/m³ 3, 600    -   (d) diameter, m 5.6 E-3 to 2.9 E-3    -   (e) Porosity, fraction 0.383    -   (f) Pore size, m 2.31 E-3 to 1.18 E-3    -   (g) Permeability (Carman-Kozeny equation) m² 2.544 E-8 to 6.625        E-9

1. A downhole formation heating system for a wellbore including a wellcasing, the system comprising: (a) a gas burner comprising a cylindricalhousing defining an intake opening and a flue opening, the housingcomprising means for receiving a supply of fuel and air; a mixing zonewhere the fuel and air are mixed; an ignition zone comprising an igniterand a reaction zone, each zone comprising a packed bed of porous media;(b) an igniter for igniting the fuel and air within the gas burner; (c)fuel and air supply tubing for delivering fuel and air to the burner;and (d) means for delivering pressurized air or an inert gas in anannular space between the well casing and the fuel and air supplytubing.
 2. The system of claim 1 wherein the porous media comprisesceramic beads.
 3. The system of claim 2 wherein the ceramic beadscomprises alumina beads.
 4. The system of claim 1 wherein the mixingzone and reaction zone comprise a pore size less than a minimumquenching distance under standard conditions of a fuel gas and theignition zone comprises a pore size greater than the minimum quenchingdistance under standard conditions of the fuel gas.
 5. A method of heattreating a formation comprising the steps of: (a) inserting a gas burnercomprising a cylindrical housing defining an intake opening and a flueopening, the housing comprising means for receiving a supply of fuel andair; a mixing zone where the fuel and air are mixed; an ignition zonecomprising an igniter and a reaction zone, each zone comprising a packedbed of porous media, into a wellbore; (b) injecting a fuel gas and airinto the gas burner to create a combustible mixture and igniting themixture to create a combustion front; and (c) causing the combustionfront to travel out the gas burner and into the formation.
 6. The methodof claim 5 wherein one zone of the gas burner has a pore size smallerthan a minimum quenching distance for an operating condition of pressureand fuel.
 7. A gas burner comprising a tubular housing adapted tooperate in a pressurized environment, the housing defining an intakeopening and a flue opening and comprising means for receiving a supplyof fuel and air; a mixing zone where the fuel and air are mixedcomprising a packed bed of porous media; an ignition zone comprising apacked bed of porous media, and a reaction zone comprising a packed bedof porous media; wherein the pore size of the mixing zone and thereaction zone is smaller than a minimum quenching distance of a fuel gasunder standard conditions while the pore size of the ignition zone islarger than the minimum quenching distance.
 8. The burner of claim 5wherein the flue opening combines with a pressure regulator forcontrolling the pressure within the gas burner above atmosphericpressure.